Last month I finished rebuilding my rear end on the 300TE

I replaced:
Diff mounts
Props haft bearing, mount and flex disks
New axle shafts

Replaced axle shaft seals on diff
Resealed rear diff cover (insides were squeaky clean)

Subframe mounts
Rear control arms

My question:

My differential has always had a whine that you can only hear when the throttle is at a certain position. It doesn't whine based on speed.

I've always ran 75w90 synthetic in it. Would a thicker fluid help at all or what part of the differential would be the cause of the noise?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k